In order to interrupt an electrical line in which a substantial current is flowing it is necessary to provide more than a simple switch to avoid arcing both while interrupting the current flow or while restoring it. Accordingly commonly owned French patent No. 2,581,790 describes such a device having a fixed contact, a movable contact, means for displacing the movable contact relative to the fixed one, and spring means urging the contacts out of contact with each other. In addition an auxiliary resistor is provided between the contacts and is set up so that its resistance increases as the movable contact is moved in the opening direction and vice versa. Thus by the time the circuit is actually opened, the inserted resistance is so large that current flow has been limited so much that arcing is impossible. Similarly as the device closes the circuit the resistance decreases until the movable contact engages the fixed contact and in effect the device has zero ohmage.
This auxiliary resistor is connected to the fixed contact and is provided along the path of the movable contact or of another contact coupled thereto for joint movement therewith and this contact slides on the auxiliary resistor. Thus the auxiliary resistor is an arcuate resistive path of graphite or ceramic charged with a doped polymer. In the closed position the wiper contact is off the resistive path and in effect sits on the fixed contact or on an element directly connected thereto.
In this arrangement the resistance is varied purely mechanically, that is the resistance inserted into the circuit being made or broken has an ohmage which is directly related only to the physical angular position of the movable contact. No factors other than position have any significant effect on the response of the device, least of all voltage or current.
It has also been suggested in U.S. Pat. Nos. 3,529,210 and 4,583,146 to employ in such devices so-called positive-temperature-coefficient (PTC) resistors whose resistance increases with their temperature which in turn is a function of the current passing through them, opposite to a thermistor. In such devices the PTC resistor is maintained in the circuit at all times so that it is subjected to considerable stress and has, therefore, a short service life.